The present invention relates to a process for forming a film of a hydrolyzed ethylene-vinyl acetate copolymer, and more particularly to a forming process of a hydrolyzed ethylene-vinyl acetate copolymer film which is provided on a surface of various plastics to give high oxygen impermeability.
A film prepared from a hydrolyzed ethylene-vinyl acetate copolymer is excellent in transparence, oxygen impermeability, and the like. So, the film is useful as various wrapping or packaging materials for foods, medical supplies, chemicals, and the like. Also, its laminate with a plastic film, a paper, a metal foil, or the like is usefully utilized.
The film of the hydrolyzed copolymer can be formed in any method. Among the various forming methods, a solution-coating method wherein a solution of the hydrolyzed ethylene-vinyl acetate copolymer dissolved in a solvent is coated on a substrate has been watched, because according to the solution-coating method, the obtained films are relatively thin in thickness, the films can be easily formed on a substrate having a complex shape such as a hollow vessel, and the coating operation can be conducted by using relatively easy painting appliances. As a solvent of the hydrolyzed copolymer used in the solution-coating method, various solvents have been proposed, and a water-containing propyl alcohol and a water-containing butyl alcohol, which are described in, for instance, Japanese Examined Patent Publication No. 46-23911 and No. 47-48489, are hopeful, since a transparent film can be obtained by using such solvents.
However, when the film is prepared from the hydrolyzed copolymer, using the above-mentioned water-containing alcohol solvent, in order to obtain a film having excellent transparence, it is required to keep a temperature of the coating solution high, i.e. 50.degree. C. in coating operation. Also, the coating solution is poor in storage stability, and it is difficult to obtain a microscopically uniform film which is excellent in film properties such as transparence and gas impermeability so far as the film is obtained by coating the solution and drying it. Accordingly, the above-mentioned method cannot sufficiently meet recent, severe requirements to qualities, and it is further required that the film properties are more improved.
In order to improve the above-mentioned defects, it has been proposed to use a solvent wherein formic acid is added to a water-containing alcohol solvent in a small amount. However, the recent, technological innovation requires to have higher properties to the films. For instance, it is required that even if the film thickness of the hydrolyzed ethylene-vinyl acetate copolymer is made thin, the oxygen impermeability is not lowered, that is, it is required that the oxygen impermeability of the film is more improved.
An object of the invention is to provide a process for forming films of hydrolyzed ethylene-vinyl acetate copolymers having more improved properties, particularly higher oxygen impermeability.
This and the other objects of the present invention will become apparent from the description hereinafter.